Add notes about supported AC/AM/LT versions, an...
Add notes about supported AC/AM/LT versions, an...

Add notes about supported AC/AM/LT versions, and add prerequsites for
AC_CANONICAL_TARGET and AC_USE_SYSTEM_EXTENSIONS.

This commit was SVN r1769.

ggouaillardet/hwloc,ggouaillardet/hwloc,shekkbuilder/hwloc,ggouaillardet/hwloc,ggouaillardet/hwloc,shekkbuilder/hwloc,shekkbuilder/hwloc,shekkbuilder/hwloc